Stem Rot ?, Hydro Set-up PIC included

See the Picture, and then here are a few details.

Any idea how to cure this problem i am having, recently scraped it off a bit and applied some baking soda to try help this out. I am running around 40% humidity, with a Fan on plans at all time, exhaust fan and fresh air intake.

Started from reputable Seed, then placed in rockwool cube.

A few details

Running 1000 HPS Light
General Hydroponics Water Farm System
Sensi Grow A and B
As well as Hobbyist Supplement Fert pack
PH 5
(in Week 3 of Veg)

tear that puck from around the base should be fine

I've seen it before in my garden. They usually die. Mine was caused by not enough circulation in the nursery, sounds like yours is dialed, so I don't know why, but it might be fucked. Try some of that black goop that they put on trees after trimming them, I can't remember what its called. Its worked for me before. It will encapsulate the stem and keep it dry, no promises on survival though bud. Sorry.